There has been a need for technology that increases the usage efficiency of wireless frequency resources in accordance with the increased demand for data communications in a variety of mobile environments and cellular wireless communications. The CDMA system, for example, is known as a system that increases the usage efficiency of wireless frequency resources by differentiating a plurality of users. In a communication system using the CDMA system, the interchannel interference caused by a spreading sequence correlation characteristic and a transmission line multipath characteristic is a primary factor that limits the frequency usage efficiency.
A system that employs Orthogonal Frequency Division Multiplexing (OFDM) involves frequency multiplexing using a sine wave and there is therefore the problem that the multipath effect is represented as fading of the signal power and separation of the transmission sine signal and multipath sine signal is problematic.
As a spreading code sequence for separating the source transmission signal and the multipath signal, a communication system that employs a complete complementary sequence, for example, has been proposed. A complete complementary sequence is a sequence that comprises an autocorrelation characteristic for which the sum of the autocorrelation functions of the respective sequences is 0 for all shifts other than a zero shift and a mutual correlation characteristic for which the sum of the mutual correlation functions of the respective sequences is always 0 for all the shifts. A ZCZ (zero correlation zone) signal without side lobes and interchannel interference and so forth is formed by using the complete complementary sequence such that the transmission signal frequency spectrum is uncorrelated. As a result, the same frequency and same time can be allocated to the pilot signal and transmission signal.
The present applicant proposed, in Patent Application No. 2002-255405, a signal design such that all of the multipath characteristic measurement signals and the plurality of data transmission signals transmitted simultaneously do not interfere with one another in transmission data modulation using a spread spectrum.
FIG. 16 is an example of a conventionally proposed signal design method. A square orthogonal matrix in which the row vectors and column vectors are orthogonal to one another is used to perform spread spectrum modulation on a multipath characteristic measurement signals An and a plurality of data transmission signals Bn, Cn, and Dn. For example, for the multipath characteristic measurement signals An, the signal array (An, An, An, An) is formed and, for the data transmission signals Bn, Cn, and Dn, the respective signal arrays (Bn, −Bn, Bn, −Bn), (Cn, Cn, −Cn, −Cn) and (Dn, −Dn, −Dn, Dn) are formed, whereby all the multipath characteristic measurement signals and plurality of data transmission signals do not interfere with one another. Further, here, a case where the square orthogonal matrix is a 4 by 4 matrix is shown.
As mentioned earlier, in a signal design such that all the multipath characteristic measurement signals and plurality of data transmission signals do not interfere with one another, a large matrix is required when transmitting a multiplicity of data transmission signals. As a result, there is the problem that the scale of the processing device required for data reception on the reception side is then large and the processing time is long and the problem that the wait time required for data transmission on the transmission side is long.
In FIG. 17, when all of the P signals of the multipath characteristic measurement signals An and the multiplicity of data transmission signals Bn to Zn are transmitted, a P×P square orthogonal matrix must be used in accordance with the number of signals P. The length of the signals in the time axis direction of the signal array formed by the square orthogonal matrix increases in accordance with the number of columns P. Therefore, the greater the number of signals transmitted, the longer the wait time required for transmission and the longer the time required for the signal reception. Further, the scale of the matching filter that extracts a predetermined signal from the reception signal also increases in accordance with the number of columns P.